The present invention relates generally to electrical water heaters. More particularly, the invention relates to methods and apparatus for pulsing electrical power to electrical resistance heating elements in a manner to improve the heating efficiency of the water heater.
A storage-type water heater typically comprises a permanently enclosed water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. The water tank has various appurtenances such as inlet, outlet and drain fittings. Additionally, the water heater is provided with a water heating and temperature control system. The water heating and temperature control system includes an electrical-resistance-heating element. The heating element extends through a fitting in the wall of the water tank such that the heating element is inside the tank. The heating element is connected to an electrical power source outside the water tank.
Conventional water heating and temperature control systems typically further include a mechanical thermostat. The mechanical thermostat closes a switch to allow electrical power through the electrical resistance heating element when water in the tank is sensed to be below a selected set point temperature, and opens the switch to stop electrical power from passing through the electrical resistance heating element when the water in the tank is at or above the set point temperature. Electrical power through the electrical resistance heating element is either fully on, passing full electrical current, or completely off. Due to variations in manufacture and hysteresis of the mechanical thermostat, the temperature of the water will xe2x80x9covershootxe2x80x9d the desired set-point temperature. In other words, the water heating and temperature control system allows the electrical resistance heating element to continue heating water in the water tank even when the water temperature is above the set point temperature. It would be beneficial to prevent or limit the amount of overshoot of the conventional water heater.
Accordingly, the invention provides a water heater having a controller for modulating electric power to an electrical-resistance-heating element in short pulses or bursts. Providing electric power to the heating element in short pulses or bursts allows an equal amount of water to be heated to a selected temperature at substantially the same rate as with a mechanical temperature controller of the prior art, yet uses substantially less electric power to heat the water. Therefore, modulating the electric power improves the efficiency of the water heater.
According to another aspect of the present invention, an even greater water heater efficiency occurs if the water heater includes multiple, controlled heating elements. If multiple heating elements are used, it is preferable that the controller activates the elements sequentially or at some predetermined frequency or in some predetermined fashion so that heat energy being transferred to the tank is distributed in a balanced or uniform manner. Even further, placing or mounting the heating elements in an approximately lower one-third volume of the heater tank enhances the energy saving performance of the water heater. Mounting the multiple heating elements in the lower one-third volume of the tank increases the radiation field of the heating elements, and results in a spreading of the wattage over a larger area. This results in an increased flow rate pattern of convection currents in the water, and helps to heat the water more uniformly.
The invention further provides a water heater including a tank for holding water, a water inlet line having an inlet opening that introduces cold water to the tank, a water outlet line having an outlet opening that withdraws heated water from the tank, a first heating element extending into the tank, and a second heating element extending into the tank. The water heater includes a control circuit operable to control the supply of electric power to the first and second heating elements in bursts, respectively. Each burst is followed by a period during which electric power is not supplied to the heating elements thereby improving the efficiency of the water heater. The control circuit is further operable to activate the first burst for a first period of time and to activate the second burst for a second period of time.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.